Make onChange prop optional, update examples and docs to treat slate as uncontrolled

[BitPhinix]

Description
Makes the <Slate/> onChange prop optional, updates the docs to treat the <Slate/> component as uncontrolled to avoid confusion.

Context
Ever since #4540 slate is in a weird half state. The API and docs suggest it's controlled as it requires a onChange and value prop and all examples keep a record of the state outside the component in a useState. This causes a lot of confusion, especially for people new to slate (see e.g. #4812) without any good reason.

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)
• You've added a changeset if changing functionality. (Add one with yarn changeset add.)

[changeset-bot]

🦋 Changeset detected

Latest commit: c520c5e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
slate-react	Patch

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